1. Field of the Invention
This invention relates to a stepping motor used to move a magnetic head of a floppy disk drive, a printer head and the like.
2. Description of the Prior Art
Conventionally well-known stepping motor structures are disclosed in Japanese Laid-Open Patent Application Nos. 47-268 and 50-140807. Those stepping motors are equipped with a circular coil wound along the periphery of a stator pole to allow the central axis of the coil to be coincident with the rotary axis of a rotor.
Stepping motors having this configuration are used in the floppy disk drive units disclosed in U.S. Pat. Nos. 3,678,481 and 4,445,155. The magnetic head of those drive units is moved to a desired location for positioning by means of a combination of the stepping motor and the lead screw.
Recently there has been a need for compact and thin floppy disk drive units, but miniaturization of the stepping motor has been a problem. To thin the drive unit disclosed in U.S. Pat. Nos. 3,678,481 or 4,445,155, the radial size of the stepping motor must be reduced. To reduce the radial size of the stepping motor of the above configuration, the diameter of the rotor or the coil thickness must be reduced. However, the torque of the motor will also be decreased. Although the torque could be increased by increasing the current flowing in the coil or by using a rotor magnet with a high magnetic energy product, it would not be practical due to cost and power consumption restrictions.
Another solution is to dispose the coil so as not to place therein the rotor as disclosed in Japanese Laid-Open Patent Application Nos. 51-7422, 53-42307 and 59-198864. However, the stepping motors disclosed in these references are single-phase motors (for watches), which are not reversible. Also, the structures of these stepping motors are complicated, and their radial sizes cannot be reduced satisfactorily.